As automotive brakes are successively applied harder and harder, a proportionately greater amount of vehicle load is shifted forward from the rear wheels to the front wheels. At high levels of braking, the shift may be great enough to cause rear wheel skidding to lock due to the lightened load on the rear wheels while failing to use the full stopping capability of the heavily loaded front wheels.
Brake proportioning valves have been developed which dynamically proportion the brake fluid pressure between front and rear brakes to achieve nearly maximum brake effectiveness at both low and high levels of fluid pressure. For example, in U.S. Pat. No. 3,771,836, a metering valve in the feed line to the front brakes of a vehicle initially reduces the amount of fluid pressure applied to the front brakes while allowing full pressure to the rear brakes. Then at higher fluid pressure the metering valve allows the full input fluid pressure to be applied to the front brakes. In Japanese Pat. application Publication No. 17617/1975, a proportioning valve in the feed line to the rear brakes initially admits full brake pressure to the rear brakes then restricts fluid pressure to the rear brakes at higher supply pressures. The net effect of the two above approaches is equivalent, namely shifting the balance of brake fluid pressures toward the front brakes for high braking levels.
Modern hydraulic brakes use tandem master cylinders in which a single mechanical pushrod input causes independent pistons in a single cylindrical bore to separately feed brake fluid pressure to front and rear wheels.
The applicant is aware of the following additional prior art patents which are hereby made of record: U.S. Pat. Nos. 3,926,476; 3,938,333; 3,252,740; and 3,462,200.